OBJECTIVE: To develop a new imaging technique of monkey hippocampus by positron emission tomography using recently found a chemical marker. RESULTS In order to determine longitudial sequela of the hippocampal damages which induced by dexamethasone during their fetal stage, we have used volumetric measurement of the hippocampal segments by MRI images in age-matched control and dexamethasone-treated juvenile monkeys. The treated groups showed consistently lower volume of the hippocampus than control monkeys. To examine the functional aspects of hippocampal neurons, we have tried to analyze the uptake of [ F-18] deoxy-glucose by PET images, but the images obtained by this marker were not specific enough to delineate the hippocampal region. The positive regions were ubiquitous in whole cortical and basal nuclei. Recently, an antagonist of 5-HT 1A receptor, WAY-100635, was found to be a specific marker for the human and macaque hippocampal gyri by PET. In our preliminary data, [Carbonyl-11] WAY-100635 showed much better and regionally specifc PET images of the hippocampus than [F-18] compound in our control monkeys. FUTURE DIRECTIONS Using this marker, we are planning to perform PET analysis of the hippocampus in 4 dexamethasone-treated and 3 control juvenile to early adult monkeys. KEYWARDS PET image, 5-HT 1A receptor, hippocampus, dexamethasone, rhesus monkey